Computer animation and other computer modeling applications may produce unwanted artifacts in a rendered image. Fireflies, or noise spikes, are an example of one type of such artifacts. Fireflies may be produced, for example, in Monte Carlo ray traced images. In a rendered image, fireflies may produce (or appear as) overly bright pixels that appear out of place in relation to neighboring (or nearby) pixels. Such a bright pixel may be considered as an outlier, in that it has an RGB luminance value that is significantly larger than what would be expected based on RGB luminance values of neighboring pixels.
FIGS. 1A and 1B illustrate examples of a rendered image 100. With reference to FIG. 1A, the fireflies 102 and 104 appear in a portion of the rendered image 100. With reference to FIG. 1B, the fireflies 102 and 104 are removed from the portion of the rendered image 100, such that visible fireflies do not appear in the rendered image 100.
Fireflies arise from low-probability events, and may be resolved in the limit as more samples are taken. However, resolving fireflies via rendering may be difficult. For example, these statistical anomalies may be so far out of an expected range that a time required for them to converge may be prohibitive, even barring numerical instabilities.
Aside from the general problem of fireflies marring a rendered image, differences in their color and variance values can cause problems for denoising solutions. For example, a distance calculation for non-local means filtering may not be robust under extreme differences in variance values.